1. Field of the Invention
The present invention generally relates to methods and apparatus of sewing and stitching. More specifically, this invention relates to a lock stitch, wherein a novel "hook and loop" style lower thread is interlocked with a conventional upper thread.
2. Description of the Prior Art
Until now, the two-thread lock stitch has been among the most widely used methods of joining fabric. Conventionally, and as shown in FIGS. 1 and 2, a two-thread lock stitch 32P includes two threads: a needle or upper thread 24, and a bobbin or lower thread 50P. The upper thread is typically wound on a spool system (not shown) to provide a continuous feed of thread. In contrast, the lower thread 50P is typically wound on a bobbin 54P to provide a predetermined feed of thread. The two-thread lock stitch 32P is considered an efficient stitch that does not unravel easily and has a "both-sides equal" aesthetic appearance. In order to maintain the aesthetic appearance, the upper and lower threads 24 and 50P must typically be composed of nearly identical size and strength material to enable stitch conformance.
Stitch conformance relates to the relative position of the upper and lower threads 24 and 50P in the stitch as shown in FIGS. 1 and 2. Conventional lock stitch practice requires a balance of stitching force on either side of a workpiece 10P being sewn, so that the lower thread 50P is not completely pulled up through the workpiece 10P. FIGS. 1 and 2 illustrate how the upper thread 24 and lower thread 50P must properly entwine at a midpoint 16P of the workpiece 10P.
Referring to FIG. 1, in operation, a needle 20 penetrates the workpiece 10P from a front side 12P thereof, carrying with it the upper thread 24 that is fed through an eyelet 22 of the needle 20. The needle 20 reaches the bottom of its stroke on a back side 14P of the workpiece 10P and starts to retract, thus forming a loop 26 from the slack upper thread 24. Referring now to FIGS. 1 and 2, and as is well known in the art, the bobbin 54P and the entire supply of lower thread 50P is encircled by the loop 26 in order to interlock the upper and lower threads 24 and 50P, thus forming the locking portion of the lock stitch 32P. The size of the bobbin 54P and quantity of lower thread 50P is necessarily relatively small to enable them to be encircled by the loop 26. Therefore, the bobbin 54P is exhausted of its lower thread 50P at extremely frequent intervals, resulting in downtime of the sewing operation, and, often, stopping and restarting of the sewing operation in the middle of the workpiece 10P.
Several alternative methods and associated devices of the prior art have been directed at mitigating the problem of the limited supply of lower thread. For example, U.S. Pat. No. 4,117,789 to Rovin et al. teaches a method of automatically loading a bobbin in situ. Rovin et al. disclose a highly complex apparatus that is capable of reloading an empty bobbin in between workpiece cycles and as an operator positions a new workpiece to the sewing machine. The apparatus refills the empty bobbin, in situ, with a precisely measured length of thread.
U.S. Pat. No. 4,140,069 to Laursen teaches a sewing method and associated apparatus for forming a double backstitch seam. The double backstitch seam is formed similarly to previous versions of two-thread lock stitches with one exception. The upper thread is fed through the workpiece and a loop thereof is formed as usual. The lower thread, however, is processed much differently than those of the prior art. The supply of lower thread is not passed entirely through the loop as usual, but instead is passed through the loop in individual thread sections equal in length to several stitches. The lower thread is fed from a relatively large continuous spool, similar to the upper thread. As the loop is formed, a free end of the lower thread is fed and sucked through the loop by a suction nozzle. As the loop is tightened by the needle retracting back through the workpiece, a looper simultaneously grabs the lower thread section near its middle and a free end of a previous lower thread section. The looper then pulls back and tightens the lower thread sections against the loop, thus completing a lock stitch.
Finally, U.S. Pat. No. 4,366,765 to Hoekstra teaches use of a combination single thread chain and lock stitch. Hoekstra discloses a stitch formation having a first loop passing through the workpiece thus forming the first half of a chain stitch. A second loop passes through the workpiece and, with the first loop, forms the second half of the chain stitch. A locking thread passes through the closed end of the second loop to form a lock stitch. The chain and lock stitches thus formed are continuously alternated for the entire length of the stitch.
In addition to the problem of a limited supply of lower thread, thread breakage is a frequent problem when generating the conventional lock stitch. If either the upper or lower thread breaks during a stitch cycle, the entire process must be stopped and the sewing machine re-threaded. Additionally, the article being sewn must be scrapped, or the stitch removed and restarted, since the stitch cannot be stopped and restarted in mid-stitch.
Therefore, what is needed is a lock stitch, method, and related apparatus that is inexpensive, efficient, does not require a bobbin having a limited supply of lower thread, that uses a method and apparatus that are relatively simple compared to the prior art, and that is not so susceptible or sensitive to thread breakage.